Amnesty said it was particularly concerned about Saudi Arabia's role in Yemen's war and that the kingdom had probably used UK-supplied jets to bomb Houthi positions, leading to civilian deaths.

Before 2009, Saudi Arabia had kept a wary eye on the six-year rebellion simmering along its southern border, led by Hussein Badreddin al-Houthi, a dissident Shia cleric, and later by one of his brothers, Abdul-Malik.

Before the Saudis entered the conflict directly last fall, rebels alleged the kingdom had provided support to Yemeni forces and even launched airstrikes of their own.

But in November, the Saudis publicly joined the fray, beating back a rebel incursion into their territory and pummeling suspected Houthi bases in northern Yemen with airstrikes.

Amnesty has "gained information" that suggests hundreds or possibly thousands of civilians diedin those bombing raids, the report said, and it is "extremely likely" that the Saudis used UK-supplied Tornado fighter-bombers to carry them out.

"The government needs to announce a thorough investigation to get to the bottom of this, reporting the finds back to parliament," Oliver Sprague, director of Amnesty's UK arms programme, said in a press release.

"Meanwhile all current and future UK supplies of arms to Saudi Arabia should be suspended pending the results of this investigation".

'Most urgent' threat

Amnesty's report also said that Yemeni security forces have killed at least 113 suspected al-Qaeda members or other "terrorists" since 2009, sometimes making no attempt to detain them first.

On Tuesday, the Washington Post posted an articlequoting senior members of the administration of Barack Obama, the US president, as saying they now consider AQAP the "most urgent threat to US security," outdoing the Yemen cell's more infamous cohorts encamped along the Afghan and Pakistani borderlands.

Coming escalation

The anonymous officials told the Post they were pushing for an "escalation" in Yemen that would include an empowered CIA presence and would "ramp up over a period of months".

US forces have already begun playing a more major role in Yemen: In December, the US "provided firepower, intelligence and other support" to Yemen as it carried out raids against several suspected al-Qaeda sites across the country, according to the New York Times.

That support included multiple US cruise missile strikes on one target, ABC News reported.

According to the Post, the US also fired a cruise missile in a botched May attackthat killed Jaber al-Shabwani, the deputy governor of the Marib province, who was reportedly meeting an AQAP member to convince him to turn himself and his men in.

Members of the White House's national security council, the Post said, have been discussing an expanded CIA presence and the potential for the use of armed, unmanned US drone aircraft in Yemen.

Hundreds of drone attacks since 2004 have proved successful at killing al-Qaeda members in the tribal areas of northwest Pakistan, but they have also stirred anger by causing civilian deaths.

A senior US military official told the Post that drones had not been used yet in Yemen only because they were needed elsewhere more urgently, although a Yemeni official said the government there probably would not ever consider allowing the United States to use them.